The objective is to utilize our recent discovery that a variety of malignant cells and polyoma virus transformed cells are more sensitive to diphtheria toxin than are normal cells in order to develop both biochemical and biological in vitro assays for the transformation function of tumor viruses. Our specific objectives and approaches to the problem are: 1. To develop further our standard biochemical assay for measuring toxin inhibition of protein synthesis in order to maximize its sensitivity in distinguishing normal from viral transformed cells. 2. To correlate the increased sensitivity to toxin of polyoma transformed BHK cells with other established in vitro characteristics of viral transformation. 3. To determine the applicability of the biochemical assay for toxin sensitivity of viral transformed cells to other cell and virus systems. 4. To use temperature sensitive (ts) mutants in order to determine if toxin sensitivity correlates with known viral gene functions. 5. To develop a rapid and easily interpreted plaque assay for viral transformation in tissue culture based on the selective toxicity of diphteria toxin for transformed cells. The immediate objectives of this study are but one aspect of the ultimate goal of our research program which is to apply toxin therapy to human neoplasms. Thus determining what types of human tumors are more sensitive to diphtheria toxin than normal human tissues and the efficacy of immunoprotection for toxin therapy in suitable animal models are outside the scope of this proposal but are being undertaken in our laboratories and will be correlated with the findings obtained in this study.